Ömer Yıldız wrote:
> In an US SENA forum, someone converted the Aux-In (marked as MP3 port in
> the photo) into an Aux-Out socket. That would also be an option if it
> cannot be done with too much know-how.

Can you please post a link to that forum thread? Maybe it's easy to do 
but too spread across that thread.

br mf

Ömer Yıldız wrote:
> My question is: I can record audio from the headphone output to an
> external recorder, but when I connect to the headphone jack, the
> speakers in the helmet are muted. So I can record the conversation, but
> I cannot communicate at the same time.

Many devices switch off the speakers while a jack is connected to the 
earphone plug.
This is done by the earphone jack which has a switch inside (resp. two 
switches for left/right speakers) and disconnects then the outgoing 
signal to the speakers.
Maybe you have the possibility to short this switch by a peace of wire 
so that it is always on. You need to be able to open your SENA device 
and find out the route of the signal from the amp to the speakers.
I think, the speaker signal is routed from the power amp to this plug 
and than further to the speakers. Connect these wires.
You should check before, which wires have a connection when the jack is 
out and if they are open, if the jack is plugged in. (Resistor 
measurement)
Use for this test a dummy plug and ideally disconnect the speakers to 
avoid wrong measurements.

Maybe the setup is different in your device, but I do not have more 
informations about it - it's only my suggestion. Perhaps a photo of the 
jack environment can help.
So, it is also possible that a controller gets only a logic signal when 
the plug is pulled in and it switches off the amp or the connection to 
the speakers. Then you need to modify this logical signal.

To amplify the weak signal for the speakers needs much more work.

Ömer Y. wrote:
> Any toughts?

Not easy.
Pic 3 shows the earbud connector. As I can see only one side of the plug 
and no PCB tracks, so it is hard to say more in a reliable statement.
The 3.5mm jack has normally 3 connectios: left, right and GND.
I can see three pads, but are there also 2 or 3 on the other side?

The switch in pic. 2 is probably a pushbutton; it is not that switch I 
meant. This is a part of the earbud connector and breaks two contacts if 
a
3.5mm jack is inserted.
Maybe these are the two left pads (and two on the other side) of the 
plug and the problem will be solved, if they are shortened. But as I am 
not shure about the details of the circuit, this may damage your device. 
So I will not advice doing this ...